Background-audio handoff for video + repository/player refactor

Hand video playback off to a native audio-only stream when the app is
backgrounded or locked, with no on-device video decode (UR-040). Adds
player_enter/exit_background_audio commands, an audio-only stream URL
for video items across the repository layer, and the frontend handoff
state machine wired into VideoPlayer. Includes accompanying
repository/offline/player refactoring and regenerates the traceability
matrix.

src-tauri/src/commands/download/smart_cache.rs

@@ -1,7 +1,9 @@
 //! Smart-cache statistics/config and album recommendation commands.
+//!
+//! TRACES: UR-045 | DR-057
 
-use std::sync::Arc;
 use log::info;
+use std::sync::Arc;
 use tauri::State;
 
 use crate::commands::{DatabaseWrapper, SmartCacheWrapper};